They are, in fact, turkey vultures. We live in Bismarck and, for the last 3 summers, we have had about 40-50 of them perched on a water tower near our house nightly. (And we live in the middle of town.) I have heard they hang out down by the river during the day. It is weird to watch them come in to roost for the evening, as they don't flap their wings much, they kind of just glide in for a landing. The first time it happened, we kind of freaked out because it started out with one or two and pretty soon the sky above our house was filled with these huge, gliding birds. It reminded me of a scene out of "The Birds". They are relatively harmless and don't make any noise (I think they can hiss?), but I guess they make quite a mess with their droppings, as they can be rather toxic to car finishes.